1. Field of the Invention
The present invention relates to a gas sensor which comprises a sensor element and measures a predetermined gas component in a measurement gas, and more particularly to an arrangement of an inner space of the sensor element.
2. Description of the Background Art
Conventionally, in order to know the concentration of a desired gas component in a measurement gas, various measuring devices have been used. As a device for measuring the nitrogen oxide (NOx) concentration in a measurement gas such as burned gas or the like, for example, a gas sensor is well known, which has an electrochemical pump cell having a structure in which a platinum (Pt) electrode and a rhodium (Rh) electrode are Banned on a solid electrolyte layer having oxygen ion conductivity, such as zirconia (ZrO2) or the like (see, for example, Japanese Patent Application Laid Open Gazette No. 8-271476). In such a gas sensor, an inner space for measuring a predetermined gas component in a measurement gas is formed in a sensor element.
In the gas sensor, the measurement of the NOx concentration in the measurement gas is conducted, for example, as follows: a pump cell provided in a first inner space communicating with an external space controls the oxygen concentration, a nitrogen oxide (NOx) is reduced or decomposed in a second inner space communicating with the first inner space, and a pump current flowing in a measuring pump cell provided in the second inner space is measured.
In the above gas sensor, however, the temperature distribution inside the sensor element is sometimes varied because of decrease in the surface temperature of the sensor element due to the effect of the decrease in the temperature, the variation in the flow velocity or the like of the measurement gas. When the temperature distribution inside the sensor element varies, the impedance or the like of the pump cell provided in the inner space varies, and consequently the measured current flowing in the measuring pump cell fluctuates though the NOx concentration is not essentially changed. In summary, the above-discussed gas sensor has the problem that the variation in the temperature distribution inside the sensor element causes fluctuation of measured values in the gas sensor.
Further, in the above-discussed gas sensor, the pumping capability of the pump cell for controlling the oxygen concentration inside the inner space depends on the effect of the distribution of the oxygen concentration or the temperature distribution inside the inner space. The decrease in the pumping capability of the pump cell raises the fluctuation of an offset current included in the measured current due to oxygen remaining near a measuring electrode, even when no nitrogen oxide (NOx) exists. Then, the fluctuation of the offset current results in an instability of the measured current flowing during the measurement of the NOx concentration. In summary, the above-discussed gas sensor has the problem that the effect of the distribution of the oxygen concentration or the temperature distribution inside the inner space causes variation of measured values.